Reducing contact resistance for field-effect transistor devices

ABSTRACT

A method and an apparatus for doping at least one of a graphene and a nanotube thin-film transistor field-effect transistor device to decrease contact resistance with a metal electrode. The method includes selectively applying a dopant to a metal contact region of at least one of a graphene and a nanotube field-effect transistor device to decrease the contact resistance of the field-effect transistor device.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is related to U.S. patent application Ser. No.13/306,357, entitled “Doping Carbon Nanotubes and Graphene for ImprovingElectronic Mobility,” and filed concurrently herewith, the disclosure ofwhich is incorporated by reference herein in its entirety.

FIELD OF THE INVENTION

Embodiments of the invention generally relate to electronic devices and,more particularly, to logic device performance.

BACKGROUND OF THE INVENTION

Graphene and Carbon Nanotubes (CNT) are candidates for replacing siliconin high and medium performance logic devices. One factor affecting theperformance of these devices is the contact resistance arising at themetal-CNT/Graphene interface. This resistance is the main contributor ofmobility degradation in short channel devices. Several solutions havebeen suggested to overcome this problem, such as using a high workfunction metal, which can lower the schottky barrier at the metal-p-typenanotube interface, thereby decreasing the resistance at the contacts.

However, such an approach depends heavily on the work function of thematerial being probed, which in the case of CNTs varies with tubediameter. For example, in the absence of a suitable high work functionmetal, it is difficult to make satisfactory contacts onto small diameterCNTs.

Another existing approach to reduce the barrier at the contacts is toincrease the electron density at the interface. This thins down thetunnel barrier present at the interface due to increase in electricalband bending, helping a direct tunneling of the electron into the CNT.Typically, such an effect is achieved using an external gate field in afield-effect transistor (FET) type structure. However, effect of gate onthe contacts may vary this depending upon FET device geometry due to thechange in electrostatics of the system. Also, gate fields are not ableto penetrate near the metal-CNT/graphene contacts due to shielding bycontact metals. This is particularly the case where metal is beneath theCNT/graphene channel (bottom contacted devices).

Accordingly, there is a need for reducing contact resistance forcontacts to CNT/graphene.

SUMMARY OF THE INVENTION

In one aspect of the invention, a method for doping a graphene andnanotube thin-film transistor field-effect transistor device to decreasecontact resistance with a metal electrode is provided. The methodincludes the steps of selectively applying a dopant to a metal contactregion of a graphene and nanotube field-effect transistor device todecrease the contact resistance of the field-effect transistor device.The decrease in contact resistance post doping is due to the increase incharge carrier concentration at the metal-CNT/graphene interface.

Another aspect of the invention includes an apparatus that includes asubstrate, a graphene and nanotube field-effect transistor devicefabricated on the substrate with an exposed contact region, wherein thecontact region is doped with a dopant, and contact metal deposited overthe doped contact region of the graphene and nanotube field-effecttransistor device.

In another aspect of the invention, graphene and CNT electricalmaterials are transferred over pre-fabricated electrodes, and theregions of the materials over the electrodes are selectively doped.

These and other objects, features and advantages of the presentinvention will become apparent from the following detailed descriptionof illustrative embodiments thereof, which is to be read in connectionwith the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a step in forming a field effect transistor (FET),according to an embodiment of the present invention;

FIG. 2 illustrates a step in forming a field effect transistor (FET),according to an embodiment of the present invention;

FIG. 3 illustrates a step in forming a field effect transistor (FET),according to an embodiment of the present invention;

FIG. 4 illustrates a step in forming a field effect transistor (FET),according to an embodiment of the present invention;

FIG. 5 illustrates another embodiment of forming a carbon nanotube FET(CNFET), according to an embodiment of the present invention;

FIG. 6 illustrates an embodiment of a dual-gate CNFET, according to anembodiment of the present invention;

FIG. 7 illustrates a step in another embodiment of forming a CNFET,according to an embodiment of the present invention;

FIG. 8 illustrates a step in another embodiment of forming a CNFET,according to an embodiment of the present invention;

FIG. 9 illustrates a step in another embodiment of forming a CNFET,according to an embodiment of the present invention;

FIG. 10 includes a graph illustrating channel resistance versus channellength for a carbon nanotube thin film transistor before and afterdoping of the contact using Ruthenium Bipyridyl Complex;

FIG. 11 includes a graph illustrating mobility versus channel length fora carbon nanotube thin film transistor before and after doping of thecontact using Ruthenium Bipyridyl Complex;

FIG. 12 is a diagram illustrating an example graphenefield-effect-transistor array, according to an embodiment of the presentinvention;

FIG. 13 is a diagram illustrating an example graphenefield-effect-transistor array, according to an embodiment of the presentinvention;

FIG. 14 is a diagram illustrating an example graphenefield-effect-transistor array, according to an embodiment of the presentinvention;

FIG. 15 includes a graph illustrating the impact of doping onpalladium/single layer graphene contact resistance, according to anembodiment of the present invention;

FIG. 16 includes a graph illustrating the impact of doping onpalladium/single layer graphene contact resistance, according to anembodiment of the present invention;

FIG. 17 includes a graph illustrating the impact of doping onpalladium/bilayer graphene contact resistance, according to anembodiment of the present invention;

FIG. 18 includes a graph illustrating the impact of doping onpalladium/bilayer graphene contact resistance, according to anembodiment of the present invention;

FIG. 19 includes a graph illustrating FET channel doping using RutheniumBipyridyl complex, according to an embodiment of the present invention;and

FIG. 20 includes a graph illustrating FET channel doping using RutheniumBipyridyl complex, according to an embodiment of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS

An aspect of the invention includes techniques for doping carbonnanotubes (CNT) and graphene to decrease the contact resistance withmetal electrodes. As described herein, resistance between CNT/grapheneand contact metal is reduced through doping of the contact regions usingvarious chemical CNT/graphene dopants. The dopant permanently increasesthe electron density at the contacts, making these regions metallic.This makes the band bending at the interface very sharp, causing thetunneling barriers to thin down, and hence decreases the contactresistance.

Doping is preferably conducted in solution phase, although gas phasedoping is also feasible. For solution processes, organic solvents suchas dichlorobenzene, dichloromethane, ethanol, acetonitrile, chloroform,methanol, butanol, among others, are suitable. Doping can beaccomplished via charge transfer from the dopants to thenano-components, for example, interaction of the lone electron pairs ofdoping molecules with the quantum confined orbitals of semiconductornanowires and nanocrystals which affects the concentration of carriersinvolved in charge transport.

With solution phase doping, for example, nano-components can be dopedbefore and/or after their integration into a circuit on a chip.Nano-components can also be doped locally on the chip using techniquessuch as inkjet printing. The doping level along a nanowire, nanotube ora nanocrystal film can be varied by masking certain portions (forexample, contacts) of the nano-component with resist and doping only theexposed portions. For device applications, nanowires can be protectedfrom damage by implementing the doping at an appropriate stage duringprocess integration.

Nanotubes, for example, carbon nanotubes, can be doped either in bulk bysuspension of the nanotubes in a dopant solution, with or withoutheating; or immersing in the dopant solution a substrate supporting thenanotubes. Although carbon nanotubes are used as examples in thefollowing discussions, doping methods of this invention can also beapplied to other semiconducting nanotubes, which may include, forexample, graphene, pentascene, fuellerence, etc, and combinationsthereof.

Interaction of carbon nanotubes with the dopants, for example, viacharge transfer, results in the formation of charged (radical cation)moieties close to the nanotubes. Bulk doping can be achieved by stirringa suspension of the carbon nanotubes in a dopant solution at a preferredtemperature from about 20 degrees Celsius (C) to about 50 degrees C.,with a dopant concentration preferably from about 1 millimole (mM) toabout 10 moles (M). Depending on the specific dopants and solvents,however, concentration ranging from about 0.0001 M to about 10 M may beused with temperatures from about 0 degrees C. to about 50 degrees C.

In general, the extent of doping depends on the concentration andtemperature of the doping medium, and process parameters are selectedaccording to the specific nano-component, dopant and solventcombination, as well as specific application needs or desired devicecharacteristics.

Device doping, that is, doping the nanotube after it has beenincorporated as part of a device structure of substrate, can be achievedby exposing the device or substrate with the nanotube to a dopantsolution. By appropriately masking the nanotube, selective doping ofportions of the nanotube can be achieved to produce desired dopingprofiles along the nanotube. As noted above, dopant concentration ispreferably in the range of about 0.1 mM to about 10 M, more preferablyfrom about 1 mM to about 1 M, and most preferably, from about 1 mM toabout 10 mM, with the solution temperature preferably from about 10degrees C. to about 50 degrees C., and more preferably, from about 20degrees C. to about 50 degrees C. With device doping, the choice ofprocess conditions also depends on compatibility with other materialspresent on the device or substrate. For example, while lower dopantconcentrations tend to be less effective in general, too high aconcentration of certain dopants may result in potential corrosionissues. In one embodiment, the doping is done under a N₂ atmospherewithout stirring or agitation of the solution. However, agitation of thesolution can be acceptable as long as it does not cause damage to thedevice.

As detailed below, FIGS. 1-4 illustrate steps of a method for forming afield effect transistor (FET). Accordingly, FIG. 1 illustrates a step informing a field effect transistor (FET), according to an embodiment ofthe present invention. A gate dielectric 120 such as silicon dioxide, oroxynitride, or a high Kelvin (K) material layer is deposited onsubstrate 100, which is generally a doped silicon substrate. In anembodiment of the invention, the silicon substrate is degeneratelydoped. The gate dielectric can have a thickness, for example, from about1 to about 100 nanometers (nm). A nano-component 140, for example,carbon nanotube, is deposited on gate dielectric 120 by spin-coating. Asused, described and depicted herein, nano-component 140 can refer to acarbon nanotube, a film of carbon nanotubes, a single CNT or CNT film,graphene flakes film or a single layer of graphene. A resist pattern isthen formed on the carbon nanotube 140 by conventional lithographictechniques. For example, a resist layer can be deposited over the carbonnanotube 140 and patterned by using e-beam lithography orphotolithography. With a positive resist, regions of the resist layerexposed to the e-beam or lithographic radiation are removed by using adeveloper, resulting in a structure with resist pattern shown in FIG. 1.

The resist pattern formed on the carbon nanotube may have one ormultiple separations from about 10 nm to about 500 nm when e-beamlithography is used, and from about 500 nm to about 10 μm withphotolithography. The multiple separations correspond to the line andspace separations resulting from the respective lithographic techniques,and represent separations between adjacent top gates. The availabilityof multiple top gates provides flexibility of individual control fordifferent logic applications, for example, AND, OR, NOR operations.

As shown in FIG. 2, a metal 160, having a thickness ranging from about15 nm to about 50 nm, is deposited on the resist pattern and overportions of the carbon nanotube 140. The metal can be Pd, Ti, W, Au, Co,Pt, or alloys thereof, or a metallic nanotube. If a metallic nanotube isused, the metal 160 may include one or more metallic nanotubes. Othermetals or alloys of Pd, Ti, W, Au, Co, Pt, can be deposited by e-beam orthermal evaporation under vacuum, while metallic nanotubes can bedeposited with solution phase techniques such as spin coating.

Following deposition of the metal, the structure can be immersed inacetone or N-methylpyrrolidone (NMP) for resist liftoff, a process thatremoves the lithographically patterned resist and the metal deposited ontop by soaking the sample in solvents such as acetone or NMP. Forexample, such solvents can also be referred to generally as resistliftoff components.

As depicted in FIG. 3, the metal portions 162 and 164 remaining on thecarbon nanotube 140 form the FET source and drain. In this embodiment,the source and drains are formed over a first and a second region,respectively, of the carbon nanotube 140, or more generally, of thenano-component 140. Following resist liftoff, the structure in FIG. 3with the carbon nanotube 140 is immersed in an organic solutioncomprising a suitable dopant as described herein in connection with anembodiment of the invention. FIG. 4 illustrates the doping moleculesbonding to the carbon nanotube 140. The doped portion of the carbonnanotube 140 (between the metal source and drain) acts as the channel ofthe FET.

FIG. 5 illustrates another embodiment of forming a carbon nanotube FET,or more generally, a FET with a channel comprising a nano-component suchas other semiconducting nanotubes, nanowires or nanocrystal films. Afterthe formation of gate dielectric 120 on substrate 100, metal portions162 and 164 are formed on gate dielectric 120 using a resist liftoffprocess (not shown) similar to that described for FIGS. 1-4. Metalportions 162 and 164, each having a thickness from about 15 nm to about300 nm, form the FET source and drain. Metals such as Pd, Ti, W, Au, Coand Pt, and alloys thereof, or one or more metallic nanotubes can beused for the metal portions 162, 164. A carbon nanotube 140, or moregenerally, a nano-component, is then disposed, for example, byspin-coating, over the gate dielectric 120 and the metal portions 162and 164. Blanket doping of the carbon nanotube 140 is achieved byimmersing the structure in an organic solution comprising a suitabledopant. The dopant molecules bond to the carbon nanotube, for example,via charge transfer interaction with the nitrogen of a dopant (forexample, such as detailed herein) donating a lone pair of electrons tothe carbon nanotube. In this illustration, the portion of the carbonnanotube 140 in contact with the gate dielectric 120 forms the channelof the FET.

Alternatively, the carbon nanotube 140 can be selectively doped througha patterned resist (not shown) that is formed over the carbon nanotube140. The patterned resist may be formed, for example, by depositing asuitable resist material over the carbon nanotube 140 and patterningusing conventional lithographic techniques. Hydrogensilsesquioxanes(HSQ), a dielectric that can be used as a negative resist, may be usedfor this purpose. Also, in an aspect of the invention, conventionalresist materials can be used such as poly(methyl methacrylate) (PMMA),etc. This is made possible due to the use of water soluble dopants suchas cerium ammonium nitrate, cerium ammonium sulfate, and rutheniumbipyridyl complex.

FIG. 6 illustrates an embodiment of a dual-gate carbon nanotube FET, ormore generally, a FET with a channel comprising a nano-component such asother semiconducting nanotubes, nanowires or nanocrystal films. Afterthe gate dielectric 120 is formed over the substrate 100, which acts asa first gate (also referred to as a bottom or back gate), a carbonnanotube, or more generally, a nano-component 140 is deposited on gatedielectric 120. Metal portions 162, 164 are formed over the carbonnanotube 140 using a resist liftoff technique such as that described inconnection with FIGS. 1-4. After metal portions 162, 164 are formed(acting as source and drain of the FET), the structure containing thecarbon nanotube 140 and metal portions 162, 164 is covered with adielectric layer 180, which can be a low temperature oxide (LTO) or achemical vapor deposition (CVD) high dielectric material such as hafniumdioxide.

A second gate 200 (also referred to as top or front gate), which caninclude a metal or highly doped polysilicon, is formed over thedielectric layer 180, for example, by first depositing a gate materialover dielectric layer 180 and then patterning to form top gate 200. Withthe top gate 200 acting as an etch mask, the dielectric layer 180 isetched such that only the portion underneath the top gate 200 remains,as shown in FIG. 6. As an example, a dilute hydrofluoric acid (HF) suchas 100:1 HF can be used as an etchant for LTO.

Additionally, the device is immersed in a dopant solution to achievepartial doping of the carbon nanotube 140. In this case, the channelincludes both the gated undoped region 500 and the two doped regions 502and 504. The doped regions 502 and 504 act like the “extensions” of acomplementary metal oxide semiconductor (CMOS) FET, resulting in reducedcontact barrier and improvements in drive current and transistorswitching. The device can be operated by either the top gate 200 or thebottom substrate 100, or both. In logic applications, it is desirable tooperate a FET with the top gate configuration for good alternatingcurrent (AC) performance.

As detailed below, FIGS. 7-9 illustrate steps in another embodiment offorming a carbon nanotube FET, or more generally, a FET with a channelcomprising a nano-component such as other semiconducting nanotubes,nanowires or nanocrystal films. After the carbon nanotube ornano-component 140 is deposited on gate dielectric 120, which haspreviously been formed over substrate 100, a patterned resist is formedon the carbon nanotube 140 using conventional lithographic techniquessuch as e-beam or photolithography.

The structure (shown in FIG. 7) containing the patterned resist andcarbon nanotube 140 is immersed in an organic solution including asuitable dopant (as detailed herein). The doping molecules bond to theexposed portions of the carbon nanotube 140. Following doping of thenanotube 140, a metal layer 160 having a thickness ranging from about 15nm to about 50 nm is deposited over the patterned resist and the dopedcarbon nanotube 140. As previously described, Pd, Ti, W, Au, Co, Pt, oralloys thereof, or one or more metallic nanotubes can be used for metal160. Metallic nanotubes can be deposited using solution phase techniquessuch as spin coating, while electron beam or vacuum evaporation can beused for deposition of other metals or alloys.

Following deposition of the metal, the structure shown in FIG. 8 isimmersed in acetone or NMP for resist liftoff. As shown in FIG. 9, metalportions 162, 164 remaining after resist liftoff form the source anddrain of the FET. The process of FIGS. 7-9 generates a significantdoping profile difference along the channel of the carbon nanotubetransistor. Note that in this case, the undoped portion (portion 500 inFIG. 6, for example) of the carbon nanotube 140 forms the channel of theFET.

To complete the formation of the FET devices illustrated in FIGS. 1-9,passivation can be performed by covering the respective devices with aspin-on organic material like poly(methyl methacrylate) (PMMA) orhydrogensilsesquioxanes (HSQ)—a low K dielectric layer, or by depositinga low temperature dielectric film such as silicon dioxide. Furtherprocessing of the device is accomplished via metallization for theback-end of the line.

In an aspect of the invention, CNT/graphene dopant solution is preparedvia mixing a number of charge transfer doping compounds in variety ofsolvents at concentrations ranging from 0.1 mM-100 mM. In one or moreembodiments of the invention, charge transfer doping compounds caninclude, for example, Cerium Ammonium Nitrate, Cerium Ammonium Sulfate,Ruthenium bipyridyl complex, and triethyloxonium hexachloro antimonate.Solvents can include water, dichloroethane, alcohols, dichlorobenzene,etc.

Additionally, in another aspect of the invention, a nanotube or grapheneFET device, fabricated on a substrate with an exposed contact region(areas with contact metal will experience evaporation), is dipped in thedopant solution. In an embodiment of the invention, the FET device canbe dipped into the dopant solution for a duration of from one second to10 hours, depending upon of the concentration of the solution used. (Astronger concentration will require a shorter the duration.) At highdoping time, the mobility of channel decreases due to the scattering bydopants. The same will happen at the contacts. The substrate is thenremoved from dopant solution and rinsed with a respective dopantsolvent. In an example embodiment, the substrate is cleaned with anamount of solvent (water, ethanol, etc.) sufficient to remove excessdopant solution (that is, to remove the unreacted dopant molecules).

In at least one embodiment of the invention, the above-noted step ofapplying dopant solution to the device can also be performed by stampingthe dopant directly over the FET channel areas. This stamping can bedone by using polymer based stamps (PDMS), ink jet printing, brushing,screen printing, etc. The dipping of the substrate in the dopantsolution ensures the doping of the contact area of the device.Thereafter, contact metal is deposited over the doped region.

Accordingly, an aspect of the invention uses newly developedwater-soluble dopants (for example, Ruthenium(III) and Cerium (IV)salts) in selectively doping the metal contact region of a carbonnanotube or graphene field effect transistor to achieve betterelectronic performance. An embodiment uses the water doping medium tomake the doping process complementarymetal-oxide-semiconductor—(CMOS)-compatible; hence, a selective regionbelow the metal contacts can be doped to decrease the contact resistanceof the field effect device. Additionally, at least one embodiment of theinvention includes using single wall nanotubes and graphene.

FIG. 10 includes a graph 1002 illustrating channel resistance versuschannel length for a carbon nanotube thin film transistor before 1006and after 1004 doping of the contact using Ruthenium Bipyridyl Complex.Graph 1002 is indicating a downward shift in the R versus L graph withcontact doping. Because the y-intercept of the line denotes the contactresistance of the FET device, a downward shift implies a decrease in thecontact resistance with contact doping.

FIG. 11 includes a graph 1010 illustrating mobility versus channellength for a carbon nanotube thin film transistor before 1012 and after1014 doping of the contact using Ruthenium Bipyridyl Complex. Graph 1010is depicting the mobility of the FET device being increased with dopingof the contacts.

The impact of charge transfer doping on the metal-graphene contactresistance is described herein. For example, doping a single layergraphene at around 6×10¹² cm⁻² does not lead to reduction inpalladium-graphene contact resistance because the number of conductionmodes in graphene underneath palladium is primarily determined by theintrinsic properties of the dipole formation and broadening of thedensity of states. On the contrary, in bi-layer graphene transistors,the contact resistance is reduced by around 40% by introducing similardoping concentration, due to the effective enhancement of the number ofconduction modes in both bilayer graphene underneath palladium and inchannel. An example embodiment of the invention includes a contactresistance of 60±20 Ω·μm at the palladium bilayer graphene interface. Inaddition, embodiments of the invention also include a doping-inducedband gap opening of 15 millielectron volts (meV) in bilayer graphenetransistors.

FIG. 12 through FIG. 14 include diagrams illustrating graphenefield-effect-transistor arrays, according to an embodiment of thepresent invention. In accordance with an aspect of the invention, theillustrations of FIG. 12 through FIG. 14 can be valid for CNTs as well.By way of illustration, FIG. 12 depicts a graphene field-effecttransistor (FET) array that includes a substrate 1104, palladiumchannels 1106 and graphene 1108, with varying channel length but withoutdoping. FIG. 13 depicts a single layer graphene FET array with varyingchannel length and doping 1112 performed before metallization. Further,FIG. 14 depicts a bilayer graphene FET array with varying channel lengthand doping 1112 performed after metallization.

FIG. 15 and FIG. 16 include graphs illustrating the impact of doping onpalladium/single layer graphene contact resistance, according to anembodiment of the present invention. By way of illustration, in FIG. 15,graph 1202 depicts the transfer characteristics of single layer grapheneFETs with (1206) and without doping (1204). The inset in graph 1202depicts the scanning electron micrograph of a typical FET array. In FIG.16, graph 1208 depicts measured contact resistance normalized to thegate bias in graphene FET arrays with (1206) and without doping (1204).The inset in graph 1208 depicts measured hole mobility in single layergraphene FET arrays with and without doping. Doping (performed aftermetallization for purposes of FIG. 15 through FIG. 18) has negligibleimpact on both mobility and contact resistance. The graphs depicted inFIG. 15 and FIG. 16 indicate that doping the channel does not change thecontact resistance, nor increase the device mobility in the case ofgraphene.

FIG. 17 and FIG. 18 include graphs illustrating the impact of doping onpalladium/bilayer graphene contact resistance, according to anembodiment of the present invention. By way of illustration, in FIG. 17,graph 1302 depicts measured contact resistance at room temperature with(1306) and without doping (1304). As used herein, V_(G) is the back gatevoltage, and V_(dirac) is the back gate voltage at which the chargeconcentration in the channel is the minimum. For graphene, this couldstill be a finite number because graphene is a semi-metal, but fornanotubes (semiconducting ones in particular) at V_(G)=V_(Dirac), thecharge concentration is zero. In FIG. 18, graph 1308 depicts thetransfer characteristics of a bilayer graphene FET with 1 μm channellength and doping at various temperatures. Line 1310 depicts 14 Kelvin(K), line 1312 depicts 100 K, line 1314 depicts 200 K and line 1316depicts 300 K. The inset of graph 1308 depicts temperature dependent offcurrent from which the doping induced band gap is inferred.

FIG. 19 includes a graph illustrating FET channel doping using RutheniumBipyridyl complex, according to an embodiment of the present invention.By way of illustration, graph 1902 depicts FET channel doping usingRuthenium Bipyridyl complex, with a dopant concentration ofapproximately 5 mM. Graph 1902 shows the doping time dependence of draincurrent. Until time <10 minutes, the doping has a positive effect onchannel current (increased by approximately 20%); however, for longertimes the current starts decreasing.

FIG. 20 includes a graph illustrating FET channel doping using RutheniumBipyridyl complex, according to an embodiment of the present invention.By way of illustration, graph 2002 depicts FET channel doping usingRuthenium Bipyridyl complex, with a dopant concentration ofapproximately 10 mM. Graph 2002 shows the doping time dependence ofdrain current. Until time <10 minutes, the doping has a positive effecton channel current (increased by approximately 10-15%); however, forlonger times the current starts decreasing.

As detailed herein, an aspect of the present invention includestechniques for doping a graphene and nanotube field-effect transistordevice to decrease contact resistance with a metal electrode, includingthe step of selectively applying a dopant to a metal contact region of agraphene and nanotube field-effect transistor device to decrease thecontact resistance of the field-effect transistor device. Such atechnique can additionally include depositing contact metal over thedoped metal contact region of the graphene and nanotube field-effecttransistor device.

In at least one embodiment of the invention, the dopant is a dopantsolution. Selectively applying a dopant solution to a metal contactregion of a graphene and nanotube field-effect transistor device caninclude placing the graphene and nanotube field-effect transistor devicein a dopant solution for a duration of time (for example, a range of onesecond to ten hours). Also, the duration of time can be based on theconcentration of the dopant being applied. If doped at a highconcentration and/or for a long duration of time, the dopant moleculecan possibly form a thick layer and prevent the electron transmissionfrom metals.

As noted herein, the graphene and nanotube field-effect transistordevice can be removed from the dopant solution and rinsed with a dopantsolvent (for example, to remove the dopant solution from non-metalcontact regions of the graphene and nanotube field-effect transistordevice with a dopant solvent).

In another embodiment of the invention, the dopant is in a gel mixturein which the gel evaporates after application. Such an implementation isparticularly effective for the process where the dopant is stamped.

Accordingly, selectively applying a dopant to a metal contact region ofa graphene and nanotube field-effect transistor device includes stampingthe dopant directly over the contact area of the graphene and nanotubefield-effect transistor device.

As detailed herein, an apparatus implementing the techniques describedabove can include a substrate, a graphene and nanotube field-effecttransistor device fabricated on the substrate with an exposed contactregion, wherein the contact region is doped with a dopant (in solutionor gel form), and contact metal deposited over the doped contact regionof the graphene and nanotube field-effect transistor device.Additionally, in another aspect of the invention, such an apparatus caninclude contact metal, a substrate, and a graphene and nanotubefield-effect transistor device fabricated on the substrate and depositedover the contact metal, wherein material areas supported under by thecontact metal are doped with a dopant (in solution or gel form). Thatis, graphene and CNT electrical materials are transferred overpre-fabricated electrodes, and the regions of the materials over theelectrodes are selectively doped.

The decrease in contact resistance post doping is due to the increase incharge carrier concentration at the metal-CNT/graphene interface.

Although illustrative embodiments of the present invention have beendescribed herein with reference to the accompanying drawings, it is tobe understood that the invention is not limited to those preciseembodiments, and that various other changes and modifications may bemade by one skilled in the art without departing from the scope orspirit of the invention.

What is claimed is:
 1. A method for doping at least one of a grapheneand a nanotube thin-film transistor field-effect transistor device todecrease contact resistance with a metal electrode, comprising:selectively applying a water-soluble dopant to a metal contact region ofat least one of a graphene and a nanotube field-effect transistor deviceto decrease the contact resistance of the field-effect transistordevice, wherein said water-soluble dopant comprises one of ceriumammonium nitrate and cerium ammonium sulfate, and wherein saidselectively applying comprises placing the at least one of a grapheneand a nanotube field-effect transistor device in a water-soluble dopantsolution for a duration of time based on the concentration of thewater-soluble dopant.
 2. The method of claim 1, further comprisingdepositing contact metal over the doped metal contact region of the atleast one of a graphene and a nanotube field-effect transistor device.3. The method of claim 1, wherein the dopant is a dopant solution. 4.The method of claim 3, wherein the dopant solution comprises a mixtureof at least one charge transfer doping compound in a solvent at aconcentration in a range of 0.1 millimole (mM) to 100 mM.
 5. The methodof claim 4, wherein the at least one charge transfer doping compoundcomprises ruthenium bipyridyl complex.
 6. The method of claim 4, whereinthe at least one charge transfer doping compound comprisestriethyloxonium hexachloro antimonate.
 7. The method of claim 4, whereinthe solvent comprises at least one of water, dichloroethane, alcohol,and dichlorobenzene.
 8. The method of claim 1, wherein the dopant is ina gel mixture in which the gel evaporates after application.
 9. Themethod of claim 1, wherein the duration of time comprises a range of onesecond to ten hours.
 10. The method of claim 1, further comprisingremoving the at least one of a graphene and a nanotube field-effecttransistor device from the dopant solution and rinsing with a dopantsolvent.
 11. The method of claim 1, further comprising removing thedopant solution from non-metal contact regions of the at least one of agraphene and a nanotube field-effect transistor device with a dopantsolvent.
 12. The method of claim 1, wherein selectively applying adopant to a metal contact region of at least one of a graphene and ananotube field-effect transistor device comprises stamping the dopantdirectly over a channel area of the at least one graphene and nanotubefield-effect transistor device.
 13. The method of claim 12, whereinstamping comprises using one of a polymer based stamp, ink-jet printing,brushing, and screen printing.
 14. An apparatus, comprising: asubstrate; at least one of a graphene and a nanotube field-effecttransistor device fabricated on the substrate with an exposed contactregion, wherein the contact region is doped with a water-soluble dopantfor a duration of time based on the concentration of the water-solubledopant, wherein said water-soluble dopant comprises one of ceriumammonium nitrate and cerium ammonium sulfate; and contact metaldeposited over the doped contact region of the at least one of agraphene and a nanotube field-effect transistor device.
 15. Theapparatus of claim 14, wherein the dopant comprises a mixture of atleast one charge transfer doping compound in a solvent at aconcentration in a range of 0.1 millimole (mM) to 100 mM.
 16. Theapparatus of claim 15, wherein the at least one charge transfer dopingcompound comprises one of ruthenium bipyridyl complex, andtriethyloxonium hexachloro antimonate.
 17. The apparatus of claim 15,wherein the solvent comprises at least one of water, dichloroethane,alcohol, and dichlorobenzene.
 18. An apparatus, comprising: contactmetal; a substrate; and at least one of a graphene and a nanotubefield-effect transistor device fabricated on the substrate and depositedover the contact metal, wherein material areas supported under by thecontact metal are doped with a water-soluble dopant for a duration oftime based on the concentration of the water-soluble dopant, whereinsaid water-soluble dopant comprises one of cerium ammonium nitrate andcerium ammonium sulfate.
 19. The apparatus of claim 18, wherein thedopant comprises a mixture of at least one charge transfer dopingcompound in a solvent at a concentration in a range of 0.1 millimole(mM) to 100 mM.
 20. The apparatus of claim 19, wherein the at least onecharge transfer doping compound comprises one of ruthenium bipyridylcomplex, and triethyloxonium hexachloro antimonate.
 21. The apparatus ofclaim 19, wherein the solvent comprises at least one of water,dichloroethane, alcohol, and dichlorobenzene.